Djokovic beat Davydenko to win the final of this prestigious end-of-year event 12 months ago and the world number three, who also defeated the Russian in the group stage last year, came from behind to frustrate him again in his opening Group B match at London's O2 Arena.

Robin Soderling's surprise victory over Rafael Nadal earlier on Monday had thrown the group wide open and Djokovic looks in the mood to mount a strong bid to retain the title.

After suffering something of a slump in the aftermath of his 2008 Australian Open triumph, Djokovic has been in supreme form of late.

The 22-year-old has reached a career-best 10 finals this year and since September's US Open he has collected three ATP Tour titles and won 18 of his 19 matches.

Djokovic's solitary defeat during that remarkable run came against Davydenko in the semi-finals in Shanghai.

Davydenko's metronomic accuracy kept Djokovic from taking control early on and the world number six got the first break in the fifth game.

There was a brief glimmer of hope for Djokovic in the next game as he earned two break points but he squandered both opportunities.

That proved the Serb's last chance of the set as Davydenko broke again in the ninth game to move in front.

After a tight start to the second set, Djokovic got the stroke of luck he needed to get back into the match.

At 4-4 and 30-0 down on Davydenko's serve, a Djokovic return clipped the net and fell kindly on the other side. Djokovic crossed himself in thanks for the good fortune and proceeded to reel off three more points to break for the first time before serving out the set.

Both men were playing with an intensity far greater than anything shown by the likes of Nadal and Roger Federer in their opening matches.

There was little to separate them and the final set followed the same pattern.

Djokovic broke Davydenko in the first game and saved three break points in a marathon sixth game.

Then Davydenko made a final push and broke as Djokovic served for the match at 5-4. But Djokovic broke back to love and held his serve this time to clinch the win in two hours and 46 minutes.